Poly(phenylene sulfide) (hereinafter often abbreviated to “PPS”) fibers have high heat resistance, chemical resistance, electrical insulating properties, and flame retardancy and have hence come to be used in industrial material applications including various filters, electrical insulators, and papermaking canvases. In particular, PPS fibers are extensively used in filter cloths for use in various industrial filters, e.g., bag filters for dust collection from discharge gases.
In the industrial material applications including such filter cloths, the PPS fibers are desired to have excellent mechanical properties. For example, PPS short fibers having high strength, an elongation as measured under 1.0 dN/dtex tensile load of 10% or less, and a degree of crimp of 12% or higher (JP-A-2004-263332) and a technique of heightening tensile strength, knot strength, and loop strength by melt-spinning PPS, subsequently stretching the spun filaments 2-7 times at a temperature not higher than the melting point of the PPS, and then heat-treating the filaments at a temperature not lower than the melting point of the PPS (JP-A-4-222217) have been disclosed.
Furthermore, PPS fibers constituted of a resin composition obtained by adding 0.1-10% by weight of aromatic hyperbranched polymer to PPS having a weight-average molecular weight of 70,000 or higher and a process of producing the PPS fibers (JP-A-2010-196187) have been disclosed. There is a statement therein to the effect that when that technique is applied, flowability of high-molecular-weight PPS is improved and the PPS is hence rendered spinnable, making it possible to obtain fibers having high strength.
Meanwhile, with respect to techniques of forming fibers of an oligomer-containing PPS resin, the following have been disclosed: a process of producing PPS fibers in which a PPS resin containing an oligomer having a dispersity ratio, represented by (weight-average molecular weight)/(number-average molecular weight), of 10 or less and a weight-average molecular weight of 1,000 or less, in an amount of 5% or less in terms of weight content is directly spun at a high speed of 3,000 m/min or higher and stretched, for the purposes of improving production efficiency and inhibiting filament breakage during spinning (JP-A-4-370218); and a process of producing PPS fibers in which a PPS resin is melted and subsequently spun through a spinneret and the spun filaments are taken up at a speed of 1,000-1,500 m/min, subsequently stretched with heating without being temporarily wound up, and then subjected to a heat treatment for relaxation at a temperature of 160-240° C. (JP-A-2000-178829).
Furthermore, PPS fibers having a dispersity ratio of 2.5 or less and an alkali metal content of 50 ppm or less and a process for producing the PPS fibers have been disclosed for the purpose of diminishing gas evolution during fiber formation steps (JP-A-2008-202164).
Moreover, a process of producing PPS fibers by a direct-spinning/stretching method and the PPS fibers have been disclosed (JP-A-2009-215680), the process being characterized in that unstretched filaments are taken up at 500-1,000 m/min, subjected to pre-stretching in a ratio of 1.03-1.09 between the take-up roll and a feed roll having a temperature of 80-100° C., stretched under specific stretching conditions, and then subjected to a constant-length treatment and a heat treatment for relaxation.
Furthermore, a process of producing PPS fibers containing impurities such as an oligomer, but have excellent production stability has been disclosed, the process being characterized by performing core/sheath composite spinning to produce PPS fibers in which the core component thereof is constituted of flash-process PPS and the sheath component thereof is constituted of quench-process PPS (JP-A-2014-25166).
However, when the techniques of JP '332 and JP '217 are applied, there has been a problem in that although strength enhancement can be attained, the PPS fibers have reduced thermal shapability due to an increase in the degree of crystallization, resulting in impaired crimpability.
The technique of JP '187 has had a problem in that the technique leads to an increase in cost and a decrease in spinnability.
When the techniques of JP '218 and JP '829 are applied, there has been a problem in that as the oligomer content increases, the process stability practically becomes worse due to the influence of the oligomers and filament breakage becomes prone to occur in the spinning and stretching steps. In addition, the fibers thus obtained tend to have considerable fluffs and impaired suitability for high-order processing.
When the technique of JP '164 is applied, there has been a problem in that although gas evolution during the fiber formation step is reduced, the fibers obtained have low strength.
When the technique of JP '680 is applied, there has been a problem in that although strength enhancement can be attained, the PPS fibers have reduced thermal shapability due to an increase in the degree of crystallization, resulting in impaired crimpability.
When the technique of JP '166 is applied, an improvement in production stability is attained as compared to when flash-process PPS is used alone, but there has been a problem in that the PPS fibers have reduced strength due to the influence of impurities, etc. as compared with the quench-process PPS alone.
It could therefore be helpful to provide a PPS fiber having high heat resistance and chemical resistance and having high strength and, despite this, has excellent suitability for high-order processing, e.g., thermal shapability because the amorphous parts thereof have high molecular movability.